So, Russell T Davies, one last question. Are the Daleks the best villains ever? "Yes. Better than anything. Better than Darth Vader. Better than Satan. My greatest joy is them working again, after all these years. Everyone said we should redesign them, and it's the one thing I dug my heels in on. That felt like madness at the time."

It wasn't the only thing that felt like madness. Putting a 40-year-old children's show at the heart of the BBC schedule was bad enough – but hiring a writer best known for Queer as Folk, an explicit drama about the Manchester gay scene?

Few would have predicted that six years later Doctor Who would be a £100 million brand, David Tennant a superstar and Davies one of the few adults not employed by Manchester United to get mobbed when he passed a playground.

In the Doctor's latest adventure, partly shot in Dubai and being shown today on BBC One, he is marooned on an alien planet with a double-decker bus and ex-EastEnder Michelle Ryan, who plays a jewel thief. Lee Evans, the comedian, is the Earth-bound academic trying to retrieve them. "It's a great big spectacular," boasts Davies, whose enthusiasm is barely dented by the fact that he is operating on four hours sleep. "A little bit Indiana Jones, a little bit Flight of the Phoenix."

And a little bit of a chance to get a tan? Not for the last time, Davies – who actually stayed behind in Cardiff – breaks into laughter.

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"People often think it's a jolly when you go abroad, but it's a nightmare. Imagine doing your job, but transplanted to a different country, and under pressure to make every penny and every second count. It's really not fun."

Almost makes you wish you had a time machine, doesn't it? But – look away now, kids – the man who sent the Doctor to meet Shakespeare doesn't believe in time travel. Even worse, he finds the idea of going back slightly boring.

"Who wants to see what happened in the past? I know what happened! The one thing I'll never know is what happens in the future. That drives me mad, it really does – in 10,000 years, what will this planet be? It's terrible not to know."

If that desire to look to the future springs from some trauma, Davies hides it well. His parents were teachers, together for 49 years and the model of understanding when he came out while at Oxford. His mother even kept her fatal cancer secret so as not to cause the family pain. "And she was right," insists Davies. "Eight years later, not one of us has ever been cross about that. We'd have driven her mad."

As a child, he would make up stories while watching Doctor Who – perfect preparation for life in children's TV, where he helped launch the career of a 15-year-old Kate Winslet, in a BBC serial called Dark Season. He harbours the urge to write more for children: "They feel stuff more keenly than anyone. If you lose your felt-tip pen when you're five, that's the biggest tragedy in the world." Soon, Davies graduated to grown-up fare, such as the ground-breaking Queer as Folk, and The Second Coming, in which Christopher Eccleston – later his first Doctor – played Jesus, reborn in modern Manchester.

He has turned down an offer to appear on Dancing on Ice, and would never have a cameo on his own show. "I despise writers who do that. They're doing someone out of work."

But despite the self-deprecating bonhomie, there's a ruthless confidence to him: he dismisses online criticism as a "carrion feast", describing the work of his team as "a magnificent achievement" that will be remembered as "a little golden age of television".

If Davies is right – and he probably is – part of the secret is that he, as the writer, has a say in virtually every aspect of the show, something normal in America but very rare here. But he criticised Dominic West, the British star of The Wire, the hit American series, who said British TV could not do contemporary drama and forced creators into making period pieces.

"I'm sure he's a very nice man. But to sit there and say that the producers of Cranford don't like making Cranford means he's an idiot, frankly. I know those people, they adore those books, and spent years trying to get that stuff on screen. We fall into this ridiculous argument of saying, 'The Wire is brilliant, therefore all UK drama is rubbish.'

"Where did that leap of logic come from? It's a playground argument." There is a major problem, though: the recession. "The BBC hasn't been able to fund itself for decades – everything's made with co-productions, and that money's disappeared. Doctor Who needs the money from international sales – every drama does – and they're all way down."

But wasn't he ahead of the curve on this, given that intergalactic bankers have been as much of a threat to his Doctor as rampaging supervillains? "I'm not anti-capitalist but you need to motivate characters. In the 60 episodes we've done, the Doctor's never described anyone as evil. All these people are driven by money, or lust, or greed. Just to say someone is evil blocks any understanding."

Emphatically not evil are his actors, who are uniformly "lovely", "nice" or "gorgeous". Tennant is, of course, the loveliest and nicest of them all, while "if I was working on series five of Doctor Who, I would bring back Michelle Ryan at the drop of a hat".

Ah – there's that "if". Davies and Tennant are about to hand over to Steven Moffat (writer) and Matt Smith, the relative unknown who will become the youngest ever Doctor. After a special in November there will be a two-part swansong at Christmas.

Already the rumours are stirring: Davies confirms that Bernard Cribbins, who played Catherine Tate's grandfather in the previous series, is returning as a full companion to the Doctor. But cameras have also captured what seems to be the Master: John Simm. Davies is coy: even if the Master is returning, he says, all might not be as it seems. "There are nightmare sequences because the Doctor's coming to the end of his time."

Given how many times the Doctor has saved the day, can Davies really raise the stakes any further? He stares at me, and tells me not to worry. "The thing is, it's such an honour to write for that man. When it comes to the last episode, there is no way I would let him down."